Characterization of progesterone receptor in bovine corpora lutea.

摘要

The steroid hormone progesterone (P₄) is secreted from the corpus luteum (CL) and has an important role in the establishment and maintenance of pregnancy and regulation of reproductive cycle length in mammals. We hypothesize that P₄ acts locally on the CL, via its genomic receptor, to regulate luteal function. The objective of Experiment 1 was to characterize mRNA expression of progesterone receptor (PR) in bovine CL during the estrous cycle. Expression of bovine luteal PR mRNA was greater (P 0.05) during the early luteal phase (days 2, 4, and 6; d 0 = estrus) than the late luteal phase (day 17). Further, PR mRNA expression was reduced (P 0.05) in day 10 luteal tissues 2 h following PGF_2α-induced luteolysis compared to other time points analyzed (vehicle, 0, 5, 15, and 30 min and 1, 2, and 12 h). The objectives of Experiment 2 were to determine if PR protein is expressed in bovine luteal cells and to determine if receptor binding affinity within luteal cells is consistent with the high luteal concentrations of P₄. Homologous competitive binding with cultured luteal cells (day 4) resulted in the identification of a specific binding component with a dissociation constant of ∼200 nM, which is 100 to 1000 fold greater than the binding affinity of PR in classical target tissues, such as mammary gland. Further, a PR antagonist, RU 486, competed for binding to the binding; component. The objective of Experiment 3 was to determine the effects of a P₄ agonist (norgestomet) or antagonist (RU 486) on P₄ production by luteinizing granulosa and luteal cells and oxytocin (OT) production by luteinizing granulosa cells, in vitro. There was no effect of treatment on concentrations of P₄ in luteinized granulosa cell conditioned media; however, in the presence of RU 486 (20 μM), DNA was decreased (P 0.05). Concentrations of P₄ in luteal cell conditioned media were also similar among the control, vehicle, and RU 486 groups following 48 h culture; however, concentrations of P₄ were decreased (P 0.05) following 48 h culture with norgestomet (20 μM). There was nc treatment effect on luteal DNA. RU 486 (0.2, 2, or 20 μM) reduced (P 0.05) concentrations of OT in luteinized granulosa cell conditioned media, and the anti-oxytocic effects of RU 486 (2 μM) were reversed by addition of norgestomet (20 μM). However, concentrations of OT were similar among the control, vehicle, and norgestomet groups. The mechanism(s) by which norgestomet decreased concentrations of P₄ is not clear, but may include nongenomic effects. Although these findings support the hypothesis that P₄ acts directly on the CL, there is no clear evidence for a direct effect on P₄ secretion via genomic PR. However, our findings do suggest that the genomic PR may be a putative mechanism by which P₄ can regulate luteal OT production.